Around 1.5 million people have yet to file their self-assessment tax returns before the January 31 deadline, Sky News has learned.

Those still to file for the 2012-13 financial year have until midnight on Friday or they risk a potential £100 fine.

According to HM Revenue and Customs (HMRC), a total of nine million paper and online returns have so far been filed.

HMRC received 326,000 returns on Wednesday, with 27,000 filed between 11am and midday alone - a rate of 450 a minute.

It was one of the busiest hours the Tax Office has ever recorded.

On Wednesday night between 10pm and midnight, 22,000 others lodged their online forms.

In recent years HMRC has bolstered its data facilities to try and keep pace with online filing increase.

HMRC's director general of personal tax, Ruth Owen, said: "There's not much time left, so make a start on your tax return today.

"Leave it too late, and you'll be hit with the penalty."

Self-assessment forms must be filed even if workers have paid any tax due for the year or no tax is liable.

Those who are already registered for self-assessment online must file by the deadline or they will be hit with an automatic £100 fine.

Taxpayers who have registered for self-assessment but have not yet set up their online profile must do so immediately and file returns as soon as possible to avoid a penalty.

People who have not yet registered for self-assessment - mainly 100,000 high-earners liable for child benefits - must contact HMRC to receive a taxpayer reference ahead of being given an online activation code.

At this point last year, 1.8 million people still had not filed their returns out of a self-assessment taxpayer base of 8.8 million people.

A total of 90.4% of people met their deadline date for filing paper returns for the 2011-12 tax year.